Scanning radar system

ABSTRACT

A scanning radar system accurately determines the lateral position of a target regardless of the heading of the target. The angle Ψ between the direction to the target and the heading of the target is calculated using equation 
     
       
         Ψ=θ−tan −1   {d  cos θ/( R−d  sin θ)} 
       
     
     and a correction value ΔX for the lateral position X is determined based on the angle Ψ.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of International application numberPCT/JP01/10918, filed Dec. 12, 2001, which in turn claims priority ofJapanese application number 2000-377841, filed Dec. 12, 2000, andJapanese application number 2001-347750, filed Nov. 13, 2001.

TECHNICAL FIELD

The present invention relates to a scanning radar system that transmitsradio waves while scanning the projection direction thereof, thatdetermines the lateral position of a target based on the strength of areflected wave returned from the target and, more particularly, to ascanning radar system that measures the lateral position of a target aswell as the distance and relative velocity thereof forvehicle-to-vehicle distance control.

BACKGROUND ART

A vehicle equipped with an FM-CW radar can measure the distance andrelative velocity of a target located ahead of the vehicle. If thedistance, for example, to a vehicle traveling ahead is to be properlycontrolled based on these measured values, the lateral position of thetarget located ahead must be determined.

A scanning radar system, as shown in FIG. 1(b), transmits a radiowavewhile scanning the projection direction thereof, and determines thelateral position of a target based on the direction in which the targetis located, i.e., the direction in which the strength of the reflectedwave from the target is the highest.

According to this method, the lateral position of the target can bedetermined accurately, provided that there is no displacement betweenthe direction to which the target is located when viewed from theradar-equipped vehicle (the direction to the target) and the directionin which the target is traveling (the heading of the target).

However, if there is a displacement between the direction to the targetand the heading of the target, the lateral position of the target cannotbe determined accurately. For example, in the case shown in FIG. 1(a) or1(c), as the power of the reflected wave is the highest at one edge ofthe target, the lateral position of the target cannot be estimatedaccurately.

DISCLOSURE OF THE INVENTION

Accordingly, it is an object of the present invention to provide ascanning radar system that can accurately determine the lateral positionof a target even when the heading of the target is displaced from thedirection to the target.

According to the present invention, there is provided a scanning radarsystem comprising: lateral position determining means for determining alateral position X of a target, based on the strength of a reflectedwave returned from the target when radio waves are projected whilescanning the projection direction thereof; means for determining anangle Ψ between a direction to the target and a heading of the target;means for determining a correction value ΔX for the lateral positionbased on the angle Ψ; and means for correcting the lateral position X bythe correction value ΔX.

Preferably, the angle Ψ determining means determines the angle Ψ, basedon a turning radius R, a distance d to the target, and an angle θbetween the direction to the target and the heading of a vehicleequipped with the radar system, by calculating equation

Ψ=θ−tan⁻¹ {d cos θ/(R−d sin θ)}

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram for explaining a problem in determining the lateralposition;

FIG. 2 is a block diagram showing the configuration of a vehicle-mountedradar system according to one embodiment of the present invention;

FIG. 3 is a diagram for explaining a method of calculating the angle Ψ;

FIG. 4 is a diagram for explaining a method of calculating the angle Ψ;

FIG. 5 is a diagram for explaining how the correction value ΔX iscalculated from the angle Ψ;

FIG. 6 is a diagram showing a second example explaining how thecorrection value ΔX is calculated from the angle Ψ;

FIG. 7 is a diagram showing a third example explaining how thecorrection value ΔX is calculated from the angle Ψ;

FIG. 8 is a diagram showing a first example explaining how ΔX iscorrected according to the distance d or turning radius R; and

FIG. 9 is a diagram showing a second example explaining how ΔX iscorrected according to the distance d or turning radius R.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 2 shows the configuration of a vehicle-mounted millimeter-wavescanning radar system as one embodiment of a scanning radar systemaccording to the present invention.

In FIG. 2, an ECU 10 calculates the turning radius R of theradar-equipped vehicle based on a signal from a yaw rate sensor 12 and asignal from a vehicle speed sensor 14, and supplies the result to anFM-CW radar 16 together with vehicle speed data. The turning radius Rcan also be calculated by using data from a steering sensor instead ofthe data from the yaw rate sensor 12. The FM-CW radar 16 projects radiowaves, in the millimeter wave region and frequency modulated by atriangular wave, in the forward direction of the vehicle and calculatesthe distance and relative velocity of a target located ahead. Further,the FM-CW radar 16 scans the projection direction of the radio waves, asearlier described, and calculates an estimated value X[m] for thelateral position of the target from the power distribution of thereflected wave. It also calculates a correction value ΔX for the lateralposition based on the data of the turning radius R, etc. supplied fromthe ECU 10, and supplies the corrected lateral position X to the ECU 10together with the distance and relative velocity data. Based on thesedata, the ECU 10 generates and outputs a control signal for maintaininga constant distance from the vehicle traveling ahead.

FIGS. 3 and 4 show the principles of methods for calculating the lateralposition X and the correction value ΔX. In FIGS. 3 and 4, R is theturning radius of the radar-equipped vehicle 20, d is the distance tothe target 22, Ψ is the angle between the direction to which the target22 is located when viewed from the vehicle 20 (the direction to thetarget) and the direction in which the target 22 is traveling (theheading of the target), and φ is the angle between the heading of thevehicle 20 and the heading of the target 22.

As can be seen from FIG. 4, the angle φ can be calculated as

φ=tan⁻¹ {d cos θ/(R−d sin θ)}

Hence, the angle Ψ is given as

Ψ=θ−φ

Since the angle Ψ represents the displacement between the direction ofthe radiowave projected to the target and the heading of the target, thecorrection value ΔX is determined in accordance with a linear functionpassing through the origin, such as shown in FIG. 5. Based on the thusdetermined ΔX, the lateral position X is corrected as

X=X+ΔX

These operations are performed using software, for example, byincorporating a CPU in the FM-CW radar.

In the function shown in FIG. 5, ΔX increases with increasing Ψ, but itis desirable that an upper bound be placed on the absolute value of ΔXas shown in FIG. 6, for reasons such as an actual vehicle width beingfinite. It is further desirable that a dead zone where ΔX is maintainedat 0 be provided centered about angle 0 as shown in FIG. 7, because itis not desirable for the lateral position X to vary for small values ofΨ due to the effect of noise. Further, when the distance to the targetis large, the correction amount ΔX should become small; therefore, ΔXshould be reduced in order to avoid the effect of noise. In this case,ΔX is multiplied by a correction coefficient that becomes smaller than1.0 when the distance d increases, as shown by the function of FIG. 8 or9, and the result is taken as ΔX. Likewise, when the turning radius R islarge, the correction coefficient is reduced in order to avoid theeffect of noise.

Since the thus calculated ΔX contains noise, it is desirable to reducethe effect of noise by correcting X by using, for example, ΔX_(n)calculated as

ΔX _(n)=(ΔX _(n−1)×3+ΔX)/4

rather than directly correcting X by using ΔX. Further, when it isjudged, from the uncorrected lateral position X, that the target istraveling in the same lane as the radar-equipped vehicle, the amount ofcorrection is reduced by multiplying ΔX, for example, by 0.7. When theamount of change of the turning radius R within a unit time becomesgreater than a predetermined value as a result of the steering action ofthe driver, the correction amount ΔX is set to 0 because it is notdesirable to correct X by responding to the change. No correction isapplied to the lateral position for a target judged to be a stationaryobject. Further, learning of a neutral position is performed in order tocancel the drift of the steering sensor (or the yaw rate sensor) basedon which the turning radius R is calculated; here, correction of thelateral position X should not be performed until the learning is done.

The calculation of the correction value ΔX has been described above, butit will be understood that, depending on the target, it is desirable notto correct the lateral position if it is found for some other reasonthat correction of the lateral position is not necessary, even when thecalculated ΔX value is not 0.

More specifically, for a target that matches any one of the followingconditions, no correction is applied because the correction isconsidered unnecessary even if the calculated ΔX value is not 0.

(1) A target that is judged, from the uncorrected lateral position X, tobe traveling in the same lane as the radar-equipped vehicle.

(2) A target from which the power (peak strength) is smaller than apredetermined fixed reference value or a reference value that varies asa function of the distance. The reason is that, in the case of a smalltarget such as a bicycle, the reflected wave (peak) is less likely to bedistributed in the angular direction.

(3) A target from which the reflected wave (peak) is not distributed inthe angular direction but exhibits a peak in only one direction.

(4) A target that has been judged to be an object, such as a largetruck, physically having a plurality of reflecting points, and for whichprocessing has been performed to take the average over a plurality ofreflected waves (peaks) giving similar distances, angles, and relativevelocities. The reason is that the angle is already corrected by theaveraging operation and, if a further correction were applied, anovercorrection would result.

For any target for which a correction has ever been made by determiningthat it does not match any one of the above conditions (1) to (4), it isdesirable to always treat such a target as a target that causes anangular displacement and apply a correction, even if the target isthereafter judged to match any one of the conditions (1) to 4).

As described above, according to the present invention, there isprovided a scanning radar system that can accurately determine thelateral position of a target regardless of the heading of the target.

What is claimed is:
 1. A scanning radar system comprising: lateralposition determining means for determining a lateral position X of atarget, based on the strength of a reflected wave returned from saidtarget when a radiowave was projected while scanning the projectiondirection thereof; means for determining an angle Ψ between a directionto said target and a heading of said target; means for determining acorrection value ΔX for said lateral position based on said angle Ψ; andmeans for correcting said lateral position X by said correction valueΔX.
 2. A scanning radar system according to claim 1, wherein said angleΨ determining means determines said angle Ψ, based on a turning radiusR, a distance d to said target, and an angle θ between the direction tosaid target and the heading of a vehicle equipped with said radarsystem, by resolving the equation Ψθ−tan⁻¹ {d cos θ/(R−d sin θ)}
 3. Ascanning radar system according to claim 1 or 2, wherein said correctionvalue determining means determines said correction value ΔX in such amanner that an upper bound value and a lower bound value are providedfor said correction value ΔX.
 4. A scanning radar system according toclaim 1, wherein said correction value determining means determines saidcorrection value ΔX in such a manner that said correction value ΔX has adead zone where said correction value ΔX does not change even if saidangle Ψ changes within a range including
 0. 5. A scanning radar systemaccording to claim 1 wherein, when the distance d to said target islarge, said correction value determining means takes a smaller value assaid correction value ΔX than when the distance to said target is small.6. A scanning radar system according to claim 1 wherein, when saidturning radius R is large, said correction value determining means takesa smaller value as said correction value ΔX than when said turningradius is small.
 7. A scanning radar system according to claim 1wherein, when said target is judged to be traveling in the same lane asthe radar-equipped vehicle, said correction value determining meanstakes a smaller value as said correction value ΔX than said determinedvalue of ΔX.
 8. A scanning radar system according to claim 1 wherein,when the amount of change of said turning radius R within a unit time isgreater than a predetermined value, said correction value determiningmeans sets said correction value ΔX to
 0. 9. A scanning radar systemaccording to claim 1, further comprising target discriminating means fordetermining, for each individual target, whether correction of saidlateral position is necessary or not, and wherein, for any target forwhich said target discriminating means has determined that correction ofsaid lateral position is not necessary, said lateral position correctingmeans does not correct said lateral position regardless of saiddetermined correction value ΔX.
 10. A scanning radar system according toclaim 9 wherein, for any target that has ever been determined as needinga correction to said lateral position, said lateral position correctingmeans always correct said lateral position.
 11. A scanning radar systemaccording to claim 9 or 10 wherein, for a target that is judged, fromthe uncorrected lateral position X, to be traveling in the same lane asthe radar-equipped vehicle, said target discriminating means determinesthat correction of said lateral position is not necessary.
 12. Ascanning radar system according to claim 9 wherein, for a target that isjudged to substantially not spread in an angular direction, said targetdiscriminating means determines that correction of said lateral positionis not necessary.
 13. A scanning radar system according to claim 9wherein, for a target for which an average angle has been taken over aplurality of reflections, said target discriminating means determinesthat correction of said lateral position is not necessary.